Update sphinx requirement from ~=8.0.2 to ~=8.1.3

Updates the requirements on [sphinx](https://github.com/sphinx-doc/sphinx) to permit the latest version.
- [Release notes](https://github.com/sphinx-doc/sphinx/releases)
- [Changelog](https://github.com/sphinx-doc/sphinx/blob/master/CHANGES.rst)
- [Commits](sphinx-doc/sphinx@v8.0.2...v8.1.3)

---
updated-dependencies:
- dependency-name: sphinx
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

pyproject.toml

@@ -101,7 +101,7 @@ sphinx = [
     # Pallets-Sphinx-Themes 2.1.1 is the first version removing old and conflicting Python 2 code.
     "Pallets-Sphinx-Themes ~= 2.1.1",
     # Sphinx 8 is the first version to drop Python 3.9.
-    "sphinx ~= 8.0.2",
+    "sphinx ~= 8.1.3",
 ]
 pytest = ["pytest >= 8"]
 test = [
@@ -126,7 +126,7 @@ docs = [
     "furo ~= 2024.8.6",
     "myst-parser ~= 4.0.0",
     # Sphinx 8 is the first version to drop Python 3.9.
-    "sphinx ~= 8.0.2",
+    "sphinx ~= 8.1.3",
     "sphinx-autodoc-typehints ~= 2.4.0",
     "sphinx-copybutton ~= 0.5.2",
     # sphinx-design 0.5.0 is the first to allow Sphinx 7.